In the manufacturing of an integrated circuit chip, one of the most important steps comprises exposing and transferring the designed pattern for the chip onto the photoresist which covers a surface of a silicon wafer. This step is also referred to as “photolithography” in which a machine called lithographic machine (or exposure machine) is used. The characteristic line width (or resolution) and the production efficiency of the integrated circuit chip are greatly affected by the resolution and exposure efficiency of the lithographic machine. The resolution and the exposure efficiency of the lithographic machine, in turn, are mainly depended on the movement precision and the working efficiency of an ultra-precise wafer moving and positioning system (which will be referred to as “wafer stage” below) which is a key system in the lithographic machine.
The basic principle of a step scan and projection lithographic machine is shown in FIG. 1. A deep ultra violet beam emitted from a light source 45 passes through a mask plate 47 and a lens system 49, so that a portion of the pattern on the mask plate is imaged onto a chip of a wafer 50. Then, the mask plate and the wafer perform synchronized movements in opposite directions at a certain speed ratio. In this way, all the patterns on the mask plate are imaged onto certain chips of the wafer.
A basic function of a wafer stage moving and positioning system is to carry a wafer during exposure and apply a movement with predetermined speed and direction to achieve precise transferring of the patterns on the mask plate to various areas of the wafer. Considering that the line width of the chip is very small (now a minimum line width of 45 nm can be obtained), to ensure a high alignment precision and resolution, the wafer stage must have an extremely high moving and positioning precision. In addition, the production efficiency of a lithographic process is significantly affected by the moving speed of the wafer stage. Thus, there is a continuous need that the moving speed of the wafer stage should be increased for improving production efficiency.
As to traditional wafer stages, such as those disclosed in EP 0 729 073 and U.S. Pat. No. 5,996,437, a lithographic machine generally has only one wafer moving and positioning unit, i.e., only one wafer stage, on which some preparation steps such as leveling, aligning and focusing are carried out. Among these steps which are all time consuming, the aligning step, which is performed by low speed scanning (with a typical alignment scanning speed of 1 mm/s) with extremely high precision, requires a very long working time. Shortening the working times for the preparation is very difficult. In this condition, for improving the production efficiency of the lithographic machine, the moving speed during stepping and exposure scanning of the wafer stage must be increased. However, increasing in moving speed inevitably results in deterioration of the dynamic properties of the system. For this reason, a lot of techniques are focused on ensuring and increasing the movement precision of wafer stages. Thus, the cost for maintaining current movement precision or obtaining even higher movement precision is significantly high.
WO 98/40791 (publication date: Sep. 17, 1998; the Netherlands) discloses a system having a dual stage structure, in which some exposure preparation steps, such as loading and unloading of the chips, pre-alignment, alignment, and the like, are performed on a second wafer stage which is simultaneously moved with a main exposure stage. Since a large amount of preparation operations for chip exposure are performed by the second wafer stage, the working time of each wafer on the exposure stage can be significantly shortened and the production efficiency can be increased correspondingly, even if the moving speed of the wafer stage is not increased. However, a main problem existed in this system is caused by its off-center driving manner (driving line being offset from the weight center of the wafer stage).
The same applicant of the present application filed an application in 2003, titled by “Ultra-Precise Wafer Positioning System for Step Projection Lithographic Machine with Dual-Stage Alternated Exposure” (application number: CN 03156436.4), in which a dual stage switching mechanism having linear guides on opposite sides is disclosed. This dual stage system does not have any spatially overlapped components, and thus no collision preventing means is needed. However, it has been found that there are still several problems in this dual stage system. First, the system requires an extremely high interfacing precision. Second, only the spaces on one side of the linear guides are used simultaneously, resulting in a bulky outer profile of the stage system, which is unwanted for the semiconductor chip manufacturers which have strict requirements to equipment space usage. Third, a bridging device with a drive is needed for switching the wafer stages of this system, which increases the complexity of the system.